A major controversy persists in cardiology as to whether true outflow obstruction may exist in patients with hypertrophic cardiomyopathy and subaortic gradients. In this study range-gated Doppler echocardiography was utilized to investigate this problem in 50 patients with hypertrophic cardiomyopathy and 20 normals. In obstructive hypertrophic cardiomyopathy, left ventricular ejection was characterized by early and rapid emptying (75\plus/minus\14% of aortic flow-velocity in the initial one-third of systole). The proportion of forward flow occurring after mitral-septal contact (and therefore concommitant with the gradient and elevated intraventricular pressure) was considerable, averaging over 40%. Mid-systolic impedance to left ventricular outflow was suggested by the rapid deceleration in aortic flow-velocity concommitant with mitral-septal contact and premature partial aortic valve closure. Furthermore, left ventricular ejection was prolonged (384\plus/minus\40 msec) and the ventricle continued to empty and shorten during the period when both the pressure gradient and markedly elevated intraventricular pressures were present. In contrast, patients with nonobstructive hypertrophic cardiomyopthy showed no evidence of impedance to left ventricular ejection. Aortic flow-velocity waveforms were similar to normals, with flow persisting to aortic valve closure; significant systolic anterior motion and partial mid-systolic aortic valve closure were absnet, and the systolic ejection period was normal (303\plus/minus\27 msec). Hence, in patients with hypertrophic cardiomyopathy mitral valve systolic anterior motion constitutes the mechanical obstruction to left ventricular emptying. Gradients produced by this mitral valve motion appear to be of patho-physiologic importance since the left ventricle continues to contract in the presence of markedly elevated intraventricular pressures.